Resilient, acrylic graft polymers are produced by a multi-stage, sequential polymerization technique which comprises alternately producing resilient and nonresilient layers around an acrylic core material. These resilient polymers are ordinarily admixed with a hard nonresilient thermoplastic methacrylic matrix resin in order to provide resiliency in articles molded from the resulting blend. Presence of the resilient acrylic graft polymer reduces susceptibility of the hard matrix resin in molded form to adverse effects resulting from impact with foreign objects.
However, the resilient acrylic graft polymers tend to cause the resulting blend to haze or whiten when articles molded from the blend are subjected to stress. This phenomenon is called stress whitening. To reduce the degree of stress whitening, Owens U.S. Pat. No. 3,793,402 suggests that the resilient, acrylic graft polymer be one in which the resilient layer both (1) surrounds a hard, nonresilient core and (2) is surrounded by a hard, nonresilient shell layer. The Owens patent indicates that if the resilient portion of the acrylic graft polymer is in the core (i.e., the first stage), such arrangement contributes to an increased tendency to whiten under stress.
Continued improvement is sought in the properties of articles molded from the blend of the acrylic graft polymer and the matrix resin. It has now been discovered that when the acrylic graft polymer of this invention is employed, articles from the blend have low stress whitening even though a resilient core is present in the acrylic graft polymer. Furthermore, at high loadings of selected acrylic graft polymer in the blend, the articles tend to have higher Gardner Impact values than those described in the Owens patent. In addition, some of the acrylic graft polymers of this invention when blended with the matrix resin tend to have higher fold endurance values than those described in the Owens patent.